I know this is late, but for future reference for people. The following is a possible work around (that should not be needed).
in the assignment group create 2 assignments/quizzes whatever. For the assignment for the one student, for the people to assign it to, get rid of everyone and add that one student.as the only person taking that assignment/quiz. then for the other assignment, assign everyone. Then in the graded book, go to the assignment that has everyone and excuse the single student for that assignment. The student getting their own assignment will see two entries in their gradebook - one excused and one with a grade. The other students will only see the single assignment.